Support for the MID RTP extension will need to be added. Support for the RtpStreamId RTP extension will need to be added. Support for setting a substream identifier on an RTP instance will need to be added, and this will need to be present on any frames received. If no SSRC mapping is found the MID and RtpStreamId should be used to determine what RTP instance the packet is for, and then an SSRC mapping added based on that.
Support for mapping not just a stream to a stream but also a stream to a substream will need to be added.
The module will need to react to RTCP information it receives and update the substream mappings accordingly if possible.
Support for the Chrome method of simulcast will need to be added, as well as the RFC implementation. Substreams will be present in the ast_sip_session_media, with 0 being the main stream. This should keep changes fairly minimal.
The "core show channel" CLI command should be extended to also show substreams.